1. Field of the Invention
The present invention relates to a half-wavelength resonator type high frequency filter, exemplified primarily by a dielectric filter used in wireless equipment such as a portable telephone.
2. Related Art of the Invention
In recent years, there has been an increasing demand for half-wavelength resonator type high frequency filters as compact and high performance filters that have superior selectively characteristics in order to achieve efficient utilization of frequencies in wireless communications. An example of a prior art half-wavelength resonator type high frequency filter will be described below with reference to drawing.
FIG. 6 shows the configuration of the prior art half-wavelength resonator type high frequency filter constructed using strip lines. In FIG. 6, reference numerals 41 and 42 are half-wavelength resonators. Reference numeral 43 is an input terminal, and 44 is an output terminal. Reference numeral 45 is an input matching circuit block Yt, 46 is an output matching circuit block Yt, and 47 is an interstage coupling capacitor Cg. The half-wavelength resonators 41 and 42 are each a strip line with both ends open, and the input matching circuit block 45Yt and output matching circuit block 46Yt are formed, for example, input and output coupling capacitors.
The operation of the thus constructed half-wavelength resonator type high frequency filter will be described below.
First, the resonators are excited at the midpoints of the respective strip lines, i.e., the dividing points between L1 and L2 and between L3 and L4, via the input and output matching circuit blocks which are, for example, input and output coupling capacitors. The interstage coupling capacitor Cg is electrically connected to both of the resonators at their resonator facets 48. The thus constructed filter exhibits a band pass characteristic with its pass band center frequency at the antiresonant frequency of the resonators and a transfer characteristic with attenuation poles formed at series-resonant frequencies of the L1 and L4 sections of the strip lines where they are equivalently grounded. In this case, if we consider only the fundamental mode, the number of attenuation poles is one per resonator.
In the above configuration, however, since each resonator resonates only at one specific frequency in the fundamental mode, the number of filter attenuation poles is limited to the number of resonators used. Further, the magnitude of attenuation is not sufficient. Another problem is that since there is a significant limitation on input/output matching, freedom in attenuation pole frequency control is limited.